Preventing accidental injury and infection from used hypodermic needles has attracted considerable interest in recent years. One way of reducing injury and infection is by retracting a needle into the syringe barrel after use. There has been a very large number of different arrangements proposed for achieving this, but it has become clear that none of these proposals is likely to become commercially acceptable unless the needle mount engaging and retraction is entirely automatic so that no additional steps have to be performed by the operator, the needle mount is securely located in the end of the syringe barrel during use, the syringe is relatively cheap and simple to manufacture yet reliable, and the volume of the dead space between the engaged plunger and needle mount is negligible.
One arrangement which has shown hope of meeting some of these requirements includes a needle mount having a deformable securing portion to disengageably retain the needle mount within one end of the barrel, means on the plunger to deform the securing portion as the plunger approaches the needle mount to disengage the engagement of the needle mount with the barrel and cooperable means on the plunger and on the needle mount to enable the plunger to engage the disengaged needle mount for withdrawal of the needle mount into the barrel when the plunger engaged with the needle mount is displaced away from the one end of the barrel.
One such proposal is illustrated in FIG. 6 of European Patent Application EP-A360313. Another such proposal is illustrated in FIGS. 7 and 8 of European Patent Application EP-A-321903, and yet another such arrangement is described in the Applicant's International Patent Application WO91/07198 (the contents of which and of corresponding patents and patent applications including U.S. Pat. No. 5,378,240 are incorporated herein by reference).
In arrangements such as these, it has been found that the deformable securing portion on the needle mount should comprise at least one resiliently flexible arm of relatively small cross section in order to ensure its ready disengagement from the barrel with minimal resistance to movement of the plunger, that is so that the disengagement occurs automatically with essentially no additional input from the operator. A problem with this is that the arm may not be very strong and may tend to be subjected to undesirable deformation resulting in disengagement from the plunger if there is any substantial resistance to withdrawal of the needle mount into the barrel with the plunger.
In EP-A-360313, the syringe barrel is described as having elastic claws which grip a needle mount located in the opening in the barrel. This grip is not released when the plunger engages the needle mount and is likely to present undesirable resistance to withdrawal of the needle mount into the barrel. Likewise, as clearly seen in FIG. 8 of EP-A-321903, there are several restrictions to withdrawal of the needle mount into the barrel with the plunger.